In communications networks employing wavelength-division multiplexing (WDM), it is advantageous for de/multiplexer(s) (hereinafter we use the general term “multiplexer”) to exhibit rectangular passbands and zero chromatic dispersion as such characteristics enhance the “cascadability” of the multiplexer. One device which has shown its value as a multiplexer, is the well-known waveguide grating router (WGR). (See, for example, M. K. Smit, “New Focusing and Dispersive Planar Component Based On An Optical Phased Array,” Electron. Lett., Vol 24, pp. 385-386 (1988); H. Takahashi, et al, “Arrayed-Waveguide Grating For Wavelength Division Multi/Deplexer With Nanometer Resolution,” Electron. Lett. Vol. 26, pp. 87-88, (1990); and C. Dragone, “An N×N Optical Multiplexer Using A Planar Arrangement Of Two Star Couplers,” IEEE Photon. Technol. Lett. Vol. 3, pp. 812-815, (1991)).
As known in the art, there are a number of ways to achieve rectangular passbands with WGRs. A first way involves the use of image mismatching, such as using a y-branch, multimode interference (MMI) coupler or short horn on the input waveguide or modifying the grating arm lengths and losses. (See, e.g., C. Dragone, “Frequency Routing Device Having A Wide And Substantially Flat Passband,” U.S. Pat. No. 5,412,744, (1995); M. R. Amersfoort, et al., “Passband Broadenting Of Integrated Arrayed Waveguide Filters Using Multimode Interference Couplers,” Electron. Lett, Vol 32, pp. 449-451, (1996); K. Okamato and A. Sugita, “Flat Spectral Response Arrayed-Waveguide Grating Multiplexer With Parabolic Waveguide Horns,” Electron Lett., Vol. 32, pp. 1661-1662, (1996); C. Dragone, “Efficient Techniques For Widening The Passband Of A Wavelength Router,” J. Lightwave. Technol. I, Vol. 16, pp. 1895-1906, (October 1998); and A. Rigny, et al., “Multigrating Method For Flattened Spectral Response Wavelength Multi/Demultiplexer,” Electron. Lett., Vol. 33, pp. 1701-1702, (1997)). Unfortunately however, image mismatching exhibits an intrinsic loss and the sharper the passband corners, the higher the loss. As can be appreciated, a rectangular passband may be viewed as N side-by-side first-order Gaussian passbands. Consequently, the transmissivity of a rectangular passband created by image mismatching must be <=1/N.
A second way to achieve rectangular passbands with WGRs is to employ synchronized gratings which have two coherently connected interferometers having the same spatial dispersion but very different free-spectral ranges. (See, e.g., C. Dragone, “Frequency Routing Device Having a Wide And Substantially Flat Passband,” U.S. Pat. No. 5,488,680, (1996); G. H. B. Thompson, et al., “An Original Low-Loss And Pass-Band Flattened SiO2 on Si Planar Wavelength Demultiplexer,” Optical Fiber Conference Digest, pp. 77, (1998); and C. R. Doerr, et al., “Compact And Low-Loss Integrated box-Like Passband Multiplexer,” IEEE Photon. Technol. Lett., Vol. 15, pp. 918-920, (July 2003)).
More recently, it has been shown that a multiplexer may be constructed from a two-arm interferometer coherently connected to a WGR, resulting in a passband of type N=2, with little or no intrinsic loss.